Oh I know such a self-directed game with much better graphics than that.

Don't let the graphics fool you, this game isn't a basic 8-bit game, it is resource intensive. It frequently uses between 1-1.5GB of RAM when running, and it frequently suffers framerate issues when exploring new chunks. Making the graphics look better would just make an already resource hungry game even worse and it would have even greater difficulty running on older PCs. Presumably, it will be optimised in beta so as not to be quite as resource-hungry as it is now, but by that time changing the graphics would be a mistake as they have come to be part of its charm. A more detailed creeper skin wouldn't be able to express the same sense of threatening sadness that the current skin does.